Conventional conductive rolls for use in electrophotographic apparatuses have a surface layer typically composed of a material containing a urethane resin as a principal component thereof and a conductive material. Recently, a low-torque motor has been employed for size reduction and cost reduction of the electrophotographic apparatuses and, if a urethane resin having an excessively high hardness is used as the surface layer material for the conductive roll, smooth rotation of the conductive roll cannot be ensured at the start up and a click sound may occur due to stick-slip. In order to overcome these problems, conductive rolls have been proposed which employ a urethane resin having a low hardness (Japanese Patent Provisional Publication Nos. 5-323777 (1993) and 3-187732(1991)).
Since the low hardness urethane resin has a relatively high friction coefficient, toner tends to thermally and physically adhere to the surface of the conductive roll, thereby easily causing toner filming. As a result, a satisfactory charging property cannot be ensured, so that image quality will be deteriorated as the number of image outputs increases. A possible method for reducing the friction coefficient of the urethane resin is to enhance the hardness or crystallinity of the urethane resin. However, the enhancement of the hardness of the urethane resin does not satisfy the requirement for the lower hardness, so that the smooth rotation of the roll at the start up may be disturbed as described above. Meanwhile, the enhancement of the urethane resin crystallinity leads to a lower solubility of the urethane resin in a solvent. Therefore, a usable solvent is limited to a highly volatile solvent, and proper application of the urethane resin is impossible. Accordingly, workablity in manufacturing the conductive roll will be deteriorated.
In order to overcome these problems, the inventors have come up with a resin composition containing a silicone-grafted acrylic polymer and a thermoplastic urethane resin, and proposed a conductive roll having a surface layer composed of the resin composition (Japanese Patent Provisional Publication No.9-305024(1997)).
The thermoplastic urethane resin contained in the surface layer material for the conductive roll disclosed in Japanese Patent Provisional Publication No. 9-305024 has a weak positive charge property and, therefore, the thermoplastic urethane resin is modified by dispersing therein the silicone-grafted acrylic polymer having a strong positive charge property. However, the silicone-grafted acrylic polymer has an insufficient compatibility with the thermoplastic urethane resin and, hence, the silicone-grafted acrylic polymer is locally present on the surface of the surface layer. This leads to aggravated toner scattering, inferior smear resistance, insufficient toner releasability, and frequent occurrence of toner filming. Against these problems further improvement is required